The separation of air to produce oxygen, nitrogen, argon, and other materials is done by distillation under low pressure to achieve power conservation. It is known that the refrigeration available from liquefied natural gas (LNG) can be utilized for cooling feed air and/or compressing component gases.
When pipelines are not feasible, natural gas is typically liquefied and shipped as a bulk liquid. At the receiving port, this liquefied natural gas (LNG) must be vaporized and heated to ambient temperatures. An efficient use of this refrigeration at the time of vaporization is highly desirable. It is becoming more common to build air separation plants with liquefiers which utilize the refrigeration available from the vaporizing LNG. An efficient scheme, which more effectively utilizes the refrigeration available from LNG to produce liquid products from air, can lead to substantial savings in energy and capital investment.
Several publications disclose the production of liquid nitrogen by indirect heat exchange against vaporizing LNG. Since the coldest temperature of LNG is typically above -260.degree. F., the nitrogen must be at a pressure greater than ambient pressure in order to be condensed because the normal boiling point of nitrogen is -320.degree. F. Typically, to condense at temperatures of about -260.degree. F., the nitrogen must be compressed to above 225 psia. Compression of the nitrogen prior to its condensation by heat exchange with LNG is one of the major sources of energy consumption in producing a liquid nitrogen product.
U.S. Pat. No. 3,886,758 discloses a method wherein a nitrogen stream is compressed to a pressure of about 15 atm (221 psia) and then condensed by heat exchange against vaporizing LNG. Since all the gaseous nitrogen is not precooled against the warming natural gas prior to compression, the amount of energy required for the nitrogen compressor is quite high.
U.K. patent application no. 1,520,581 discloses a process of using the excess refrigeration capacity associated with a natural gas liquefaction plant to produce additional LNG, specifically for the purpose of providing refrigeration for the liquefaction of nitrogen. In the process, the nitrogen gas from the air separation plant to be liquefied is compressed without any precooling with LNG.
Yamanouchi and Nagasawa (Chemical Eng. Progress, pp 78, July 1979) describe another method of using LNG refrigeration for air separation. Once again, nitrogen at about 5.2 atm is compressed to about 31 atm without any precooling. Moreover, in this paper, LNG is vaporized in the LNG heat exchanger at close to ambient pressure (15 psia).
U.K. Pat. No. 1,376,678 teaches that evaporation of LNG at close to atmospheric pressure is inefficient because the vaporized natural gas must be admitted into a distribution pipeline at a pressure at which it can reach its destination, i.e., a transport pressure. This transport pressure is much higher than atmospheric pressure usually not exceeding 70 atm (1029 psi). Therefore, if LNG is vaporized at atmospheric pressure, then a considerable amount of energy is required to recompress the vaporized gas to its transport pressure. As a result, in U.K. Pat. No. 1,376,678, the LNG is first pumped to the desired pressure and then vaporized. Unfortunately, the process of refrigeration energy recovery taught in this patent is inefficient because not all of the refrigeration available from the LNG is recovered and the vaporized natural gas leaving the LNG heat exchanger is still quite cold (-165.degree. F.). This incomplete recovery of refrigeration implies that, for this process, large quantities of LNG will be required to produce the desired quantity of liquid nitrogen. Japanese patent publication no. 52-37596 (1977) teaches vaporizing low pressure LNG against an elevated pressure nitrogen stream, which is obtained directly from a distillation column which operates at an elevated pressure. In the process, only part of the LNG is vaporized against the condensing nitrogen and the remainder of the LNG is vaporized in the other heat exchangers; this is an inefficient use of the refrigeration energy of LNG. The vaporized natural gas is then compressed.
U.S. Pat. No. 3,857,251 discloses a process for producing liquid nitrogen by extraction of nitrogen from the vapors resulting from the evaporation of LNG in storage tanks. The gaseous nitrogen is compressed in a multistage compressor with interstage cooling provided by water, air, propane, ammonia, or fluorocarbons.
Japanese patent publication no. 46-20123 (1971) teaches cold compression of a nitrogen stream which has been cooled by vaporizing LNG. Only a single stage of nitrogen compression is used. As a result, an effective use of LNG cold energy, which vaporizes over a wide range of temperature, is not obtained.
Japanese patent publication no. 53-15993 (1978) teaches the use of LNG refrigeration for the high pressure nitrogen drawn off the high pressure column of a double column air distillation system. The nitrogen is cold compressed in a multistage compressor, but without any interstage cooling with LNG.
German Pat. No. 2,307,004 describes a method for recovering LNG refrigeration to produce liquid nitrogen. Nitrogen gas from the warm end of a cryogenic air separation plant is close to ambient pressure and ambient temperature. This feed nitrogen is compressed, without any LNG cooling, in a multistage compressor. A portion of this compressed gas is partially cooled against LNG and expanded in an expander to create low level refrigeration. The other portion of compressed nitrogen is cold compressed and condensed by heat exchange against the expanded nitrogen stream. The expanded gas is warmed and recompressed to an intermediate pressure and then fed to the nitrogen feed compressor operating with an inlet temperature close to ambient. It is clear that most of the nitrogen compression duty is provided in compressors with inlet temperature close to ambient temperature and that no interstage cooling with LNG is provided in these compressors.
U.S. Pat. Nos. 4,054,433 and 4,192,662 teach methods whereby a closed loop, recirculating fluid is used to transfer refrigeration from the vaporizing LNG to a condensing nitrogen stream. In U.S. Pat. No. 4,054,433, a mixture of methane, nitrogen, ethane or ethylene and C.sub.3 + is used to balance the cooling curves in the heat exchangers. The gaseous nitrogen from the high pressure column (pressure.TM.6.2 atm) is liquefied without any further compression. However, a large fraction of nitrogen is produced at close to ambient pressure from a conventional double column air distillation apparatus. Its efficient liquefaction would require a method to practically compress this nitrogen stream, which is not suggested in this U.S. patent.
In U.S. Pat. No. 4,192,662, fluorocarbons are used as recirculating fluid wherein it is cooled against a portion of the vaporizing LNG and then used to cool low to medium pressure nitrogen streams. This scheme presents some problems and/or inefficiencies. Energy losses due to fluorocarbon recirculation are large; requiring additional heat exchangers and a pump. Furthermore, the use of fluorocarbons has negative environmental implications and use of alternate fluids are expensive.
Japanese patent publication no. 58-150786 (1983) and European patent application no. 0304355-A1, (1989) teach the use of an inert gas recycle such as nitrogen or argon to transfer refrigeration from the LNG to an air separation unit. In this scheme, the high pressure inert stream is liquefied with natural gas, and then revaporized in a recycle heat exchanger to cool a lower pressure inert recycle stream from the air separation unit. This cooled lower temperature inert recycle stream is cold compressed and a portion of it is mixed with the warm vaporized high pressure nitrogen stream. The mixed stream is liquefied against LNG and fed to the air separation unit to provide the needed refrigeration and then returned from air separation unit as warm lower pressure recycle stream. Another portion of the cold compressed stream is liquefied with heat exchange against LNG and forms the stream to be vaporized in the recycle heat exchanger. These schemes are inefficient. For example, all of the recirculating fluids are cold compressed in a compressor with no interstage cooling with LNG.
Cold compression of air is described in Japanese patent application nos. 53/124188-A and 51/140881. In both disclosures, feed air is cooled by direct heat exchange, i.e., air and LNG are fed through the same heat exchanger. This seems to reduce power consumption for the main air compressor. However, their flow passages appear adjacent to one another. If the pressure of the LNG were higher than that of the ambient air, then any leakage of hydrocarbons to the air stream would present an explosion hazard in the downstream air separation unit cold box. In fact, the feed air pressure to the air separation unit is usually less than 100 psia, while vaporized LNG is greater than 500 psia.